30F THE RETURN RATES OF TERRITORY-HOLDING WILLOW WARBLERS - 1990

by David Norman

Summary

From 1981 to 1990. 98 adult Willow Warblers were caught on territory on No.3 Bed at Woolston Eyes. From 1981 - 84, 41% of males returned to hold territory in the year following ringing, similar to published figures for the average annual survival rate from elsewhere, but the return rate dropped greatly from 1985 onwards. The likely reasons are twofold: i) a decline in the habitat quality such that the area continued to be occupied, but birds were not faithful to the site from year to year; and ii) a real decline in the year=to-year survival of the species, coinciding with the national drop in population from 1985 onwards.

Method

Territory-holding Willow Warblers were ringed on No.3 Bed in April and May every year from 1981 to 1990 inclusive. Each bird was given a single colour-ring corresponding to the year of capture. Birds were caught by erecting a mist-net in their territory and playing a tape lure for limited periods: most birds were caught very quickly. This activity conforms to BTO guidelines and there is no evidence of any effect on breeding performance. Most of the birds caught were males; females tend not to respond to the lure. Birds caught as part of general mist-netting operations were not included in this analysis because of their uncertain status: some of them were likely to have been on passage through the site rather than territorial residents.

Results

A total of 89 different male Willow Warblers was caught holding territory on No.3 Bed in ten years, with 9 females. The table below lists the numbers of males ringed each year, together with those retrapped from the previous year, giving the total number of ringed birds: in the three years marked *, birds were also caught that had been ringed at least two years previously. Also shown is the number of singing males counted on No.3 Bed during the annual warbler survey, conducted in mid-May each year, according to the annual Woolston reports.

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

Number ringed 12 8 9 11 5 2 5 22 7 8

No. retrapped from previous year 7 1 4 1 0 1 0 0 0

Total No. of ringed birds 12 15 11* 15 6 2 7* 23* 7 8

Warbler survey singing males 11 16 12 15 22 21 22 18 29 17

In the first four years of study, 1981-84, coverage was thorough and almost every territorial male on No.3 Bed was ringed. After that, the intensity of effort declined and, apart from in 1988, only a sample of the singing birds was caught. In 1981 and1988, more birds were ringed than the total counted singing on the day of the warbler count.

Fourteen male Willow Warblers were retrapped the year after ringing. In the three years 1982-84 inclusive, 12 of 29 ringed birds were retrapped in the year following ringing (41%), while in the following six years, 1985 - 1990, only 2 of 52 birds were retrapped in the year following ringing (4%). Two birds were caught two years after ringing, and one bird was retrapped five years after first holding territory. This male was originally ringed as a juvenile on 11 July 1982, was caught again on terrirtory on 31 May 1983 then retrapped on 26 April 1988, aged almost six years. Only nine female Willow Warblers were ringed, and only one of them was retrapped in a subsequent year. She was ringed as a breeding adult on 18 May 1985 and caught again on 6 May 1990, also almost six years of age. She had presumably been present on No.3 Bed every year, but had not been seen in the four intervening years. The British longevity record-holder, out of almost 1,200,000 Willow Warblers ringed, lived 10 years 8 months from ringing to final report.

Discussion

Analysis od survival/mortality using mark-recapture techniques usually assume that adult birds, if they survive, normally return to breed on or near the same territory from year to year: adult dispersal is low. In these circumstances the retrapping rate equals the survival rate, provided that every bird is caught. In the first four years of this study (1981-84), every male Willow Warbler known to have held territory on No.3 Bed was indeed caught - as shown by the similarity between the totals of birds handled and the warbler survey results - and the return rate of 41% is probably a reasonable estimate for the birds' year-to-year survival.

From a study in Hertfordshire, 1981-89, the average annual adult survival rate of Willow Warblers was calculated to be 47% (Pratt & Peach 1991), and at a group of adjacent sites in Surrey, 1987-93, the minimum average annual survival rate for the species varied from 31% to 41% (Lawn 1994). The value from the early years of this Woolston study is similar to these figures. It is odd, however, that only one bird was caught in three successive years when almost one-in-five of them should still be alive.

From 1985 onwards the intensity of my study waned, but the number of retraps dropped precipitately. Too few birds were retrapped to allow any calculation of survival, but it certainly seems that few male Willow Warblers returned to breed on No.3 Bed. Lawn (1994) showed that site fidelity of territorial males varied significantly across sites, probably related to the quality of the territory. Differences in return rates between different parts of his Surrey site were largely attributable to differences in site fidelity rather than differential survival. This could also apply at Woolston if the birds' perception of No.3 Bed changed over the years, and the low retrap rate is because surviving birds chose to establish territories elswhere. The population od Willow Warblers on No.3 Bed actually increased somewhst during the second half of the 1980s (see table above) but perhaps the quality of the habitat was declining: certaily the progression towards more wooded vegetation, rather than the species' favourite pioneer scrub, was well under way. Unfortunately, I did not map the birds territories and do not know whether the same terrotories were used from year to year. It wuld also have been desirable to include some measure of habitat, to attempt an assessment of territory quality.

Although this work does not prove that the survival of Woolston birds has changed, it is known that Willow Warbler populations nationally declined greatly, with the largest drop from 1984 - 1992 (http://www.bto.org/birdtrends2005/wcrwilwa.htm). This decline has been explained by a fall in survival rates (Peach et al 1995). The two main factors are thought to be pressures on migration and in the winter, and a reduction in habitat quality on the breeding grounds (Fuller et al 2005).